The present invention relates to the sensing of particles in the nanometer to micrometer size range, and more particularly to instruments that facilitate droplet formation and growth on particles and other elements suspended in aerosols.
A well known technique for detecting submicrometer particles (or other suspended aerosol elements, e.g. droplets) is to “grow” the elements into larger droplets through condensation of the vapor of a working fluid, thus to enhance optical detection. For example, in one form of continuous flow condensation particle counter or CPC (sometimes referred to as a condensation nucleus counter), an aerosol stream of submicrometer particles suspended in air or another gas is directed through a saturation region, where the working fluid evaporates into the aerosol stream. Upon leaving the saturation region, the aerosol stream is cooled to supersaturate the working fluid vapor. This causes the vapor to condense onto the particles, forming aerosol droplets considerably larger than the particles. The aerosol stream including the droplets is directed past an optical detection system, usually employing a laser beam and associated optics to individually sense the droplets based on scattering or interruption of the coherent energy. The droplet or particle count can be used to determine a particulate concentration in the air or other gas. Following the detection, the droplets or particles (after evaporation) may be collected for chemical analysis. For further information regarding this type of instrument, reference is made to U.S. Pat. No. 4,790,650 (Keady).
In a similar instrument known as a mixing-type CPC, an aerosol stream saturated with a working fluid vapor is mixed with a sample gas provided at a temperature lower than the aerosol temperature. This cools the gas mixture sufficiently to achieve supersaturation. As a result, particles in the gas mixture act as heterogeneous nucleation sites for condensation of the working fluid, growing droplets large enough for optical detection. Examples of this type of CPC are disclosed in U.S. Pat. No. 5,903,338 (Mavliev, et al.).
In another alternative condensation particle counter, described in U.S. Pat. No. 6,712,881 to Hering and Stolzenburg, an aerosol stream is conditioned to a desired temperature and optionally saturated, then provided to a higher-temperature condenser whose internal walls are wetted with a working fluid, preferably water. As the aerosol stream flows along the condenser, the working fluid vapor diffuses from the walls into the stream and condenses onto the particles.
Water is the preferred working fluid because of its nontoxicity, low cost, and lack of unpleasant odor. Further, water is preferred in systems that involve post-detection chemical analysis of the particles, especially in the case of biomolecules.
In conventional laminar flow condensation particle counters, working fluids with mass diffusivities lower than their thermal diffusivities are preferred, because the aerosol is cooled rather than heated as it enters the condenser. Accordingly, preferred alternative working fluids include butanol, isopropanol, and glycerol, with butanol perhaps being the most preferred. The mass diffusivity of butanol vapor, for example, is less than one-third that of water. Attempts to use water in conventional, cooled-condenser laminar flow condensation particle counters have not succeeded, although mixing-type condensation particle counters have been designed to use water as the working fluid.
In any instrument that relies on condensation to grow particles into larger droplets, the effectiveness of the technique depends on particle size and degree of supersaturation. For a given supersaturation, there is a minimum particle diameter capable of supporting growth into a droplet, commonly known as the threshold diameter. The degree of supersaturation can be controlled by controlling aerosol flow rates and temperatures along the saturation region and condenser or growth tube.
In spite of the desirable properties of water and its suitability in the Hering/Stolzenburg and mixing-type particle counter designs, its use brings to light another problem. When the working fluid is water, another variable becomes significant: the particulate material's affinity to water. Assuming that supersaturation is constant, the threshold diameter varies, from smaller threshold diameters for hydrophilic materials to larger threshold diameters for hydrophobic materials. As a result, particle counts are variable not only as a function of particle concentration, but also as a function of the character or makeup of the particles. In situations where the particle composition is unknown, or where proportions of different known particles may vary, this imposes a significant limitation on the use of water as the condensate in particle detection instruments.
Therefore, it is an object of the present invention to provide an aerosol analyzing instrument in which the formation and growth of working fluid droplets is more uniform, despite differences among suspended particles and other elements in terms of their affinity for the working fluid.
Another object is to provide a condensation particle counter capable of employing water as the working fluid, with improved sensitivity for measuring hydrophobic aerosol elements in the nanometer to micrometer diameter range.
A further object is to provide a process for modifying an aerosol to enhance droplet formation and growth on the particles and other elements suspended in the aerosol.
Yet another object is to provide a condensation particle counter capable of generating particle counts that more accurately reflect particle concentrations while minimizing the influence of extraneous factors such as particle or element makeup and affinity to the working fluid vapor.